#1282 prosodyctl check: Highlight "unused" components

Description of feature:
`prosodyctl check config` should identify Components that are defined in the config file that will not show in any host's disco#items.
Motivation: (Why?)
People sometimes put components under a sub-sub-domain, or have a VirtualHost like "jabber.example.com" and a Component like "myservice.example.com".
Such setups ought to use disco_items if it isn't possible to rename the components.